[adjective] [before noun], [adverb] [not gradable] [specialized] - sung by a group of people without the help of any musical instrumentsThe choir's poor ensemble is disastrously exposed in the a cappella passages.Barber-shop choirs sing a cappella.
A CAPPELLA
Meaning of A CAPPELLA in English
Cambridge English vocab. Кембриджский английский словарь. 2012